Before the announcement, Obama sent an early morning email to supporters, saying he had devoted a "considerable amount of time and deliberation to this decision" and consulted with outside experts and groups.

That will be a hard sell because Republicans control the Senate, which must confirm any nominee, and the party's leaders want to leave the choice to the next president, denying Obama a chance to alter the ideological balance of the court before he leaves office next January.

Republicans contend that a confirmation fight in an election year would be too politicised.

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In advance of Obama's announcement, the Republican Party set up a task force that will orchestrate attack ads, petitions and media outreach.

The aim is to bolster Senate Republicans' strategy of denying consideration of Obama's nominee.

The party's chairman, Reince Priebus, described it as the Republicans' most comprehensive judicial response effort ever.

Garland was twice before considered as a nominee for the lifetime job as a justice but passed over when Obama chose Sonia Sotomayor and Elena Kagan.

Garland had previously served in the Justice Department under Democratic president Bill Clinton.

Garland has won praise in the past from Republicans and Democrats and is viewed as a moderate whose legal approach was shaped by his lengthy career as a federal prosecutor.